The present invention refers to a remote activation mechanism for equipment hold down and release, specially applicable to the release of systems and equipment requiring a firm structural hold down, such as deployable antennas, solar panels, deployable radiators, as well as ejectable equipment (such as covers) of space vehicles, oceanographic equipment, aeronautics, military equipment, etc.
There are units or appendages needing remote release activation. A clear example, although not the only one, is the release of satellite appendages that require generally to be firmly joined to the satellite structure to survive the launch loads, while they require to be in a different configuration to be operative, decoupled, far away from the satellite main structure, or disconnected from the satellite structure. Appendages such as antennas, solar arrays, thermal radiators, experiments, ejectable protection covers etc., need to be released and deployed or ejected once the vehicle is in orbit.
Up to now, the problem of the release of such equipment has been solved using pyrotechnic release systems, such as cable/bar cutters, pushers and releasable appendage separation nuts. Those systems are based on the activation of a cutting, pushing, or separation mechanical system, using the pressure of the gases produced when a small explosive charge is detonated. Those actuators have been used extensively due to its compactness, simplicity and reliability. Nevertheless, those systems have the following inconveniences:
a) They can produce uncontrolled residue, such as metallic particles, polluting gases, etc. that in many cases cannot be afforded due to the type of systems the satellite contains, such as optical equipment, precision mechanisms, electronic equipment, etc.
b) They produce very high accelerations at high frequencies, which shock the main structure, transmitting this excitation to all the equipment located at the surroundings. Those accelerations at high frequencies are the main characteristic of explosive actuators, being extremely dangerous for the integrity of delicate equipment such as radio frequency and electronic equipment.
Those accelerations have two main causes. The first one is the explosion itself; the second is the sudden energy deformation liberation originated by the necessary preload of the hold down system. It is necessary to add that the achieved acceleration level is hardly attained by analysis due to the complexity of the model definition to represent the real event, and due to the uncertainty in the definition of the parameters involved in its simulation. Due to that, expensive tests on model representing the reality should be necessary.
c) Pyrotechnic systems are not reusable. They demand the complete substitution of the actuator to be able to maintain the system at the same operative level than before, as well as a new re-connection of the electric connectors necessary to provide the initiator activation current. That makes that in a satellite qualification test campaign to be performed before being launched and before being operated in orbit, could need of great quantity of pyrotechnic devices.
d) Pyrotechnic systems have an intrinsic risk during their manipulation, storage, or operation, being necessary to take care of potential electrostatic discharges and of potential electromagnetic interference that could activate those systems.
e) Pyrotechnic systems have not the capacity of being proven functionally before being used to achieve a confidence in their operation, being necessary the use of statistical data or test by lot on randomly selected units. Therefore, the pyrotechnic device that will be used in orbit could not be used on ground for test purposes, being impossible to perform a direct verification of the unit. Additionally, the connectors providing electric current to the pyrotechnic initiators should be disconnected and re-connected in each operation, meaning a significant increase of failure risk in that electrical connection.
f) The pyrotechnic systems are based on the detonation of an explosive material. This detonating material is usually provided with a limited life due to the degradation of the stability of explosive chemical composition with time.
An alternative to pyrotechnic systems, used in some cases, is the use of motorised hold down and release mechanisms. Those mechanisms are usually complex and heavy, suffering of lack of compactness, high cost, high sensibility to the environmental conditions, and low reliability.
The current tendency in the satellite construction is towards smaller vehicles with more sensitive instrumentation. The combination of highly sensitive equipment, in smaller and slighter structures in combination with hold down release mechanisms makes necessary the development of non explosive release actuators for not endangering the survival of the mission.
These inconveniences have given way to non-explosive hold down release mechanisms, based on three different concepts:
a) Mechanisms with a fusible wire, based on providing the sufficient heating energy to a wire that maintains the mechanism preload up to fuse it (or at least weaken it up to rupture). The rupture or fusion of that wire makes as a trigger that activates the mechanism that performs the release. Once the wire breaks or fuses and the mechanism is operated, the system should be rearmed and the initiator containing the fused or broken wire should be substituted to have the device ready for a new operation.
b) Mechanisms with paraffin actuators, based in the paraffin increase of volume when it fuses (15% approximately). This increase of volume is controlled as to push a stem that can activate the release. These mechanisms are voluminous, complex, expensive and very slow in their performance, being able to present problems of leakage or molecular contamination in vacuum environment.
c) Mechanism with shape memory alloy piece, based in the use of the significant contraction or expansion of shape memory alloys when the piece reaches its transition temperature, activating directly the release, by breaking the joining bolt or by changing the geometry of the element maintaining the joint sufficiently as to release the fastener or the retainer.
Pieces manufactured with shape memory alloys, being processed in a suitable way and under certain circumstances, have the property of being able to change their form cyclically with their temperature, being able to reach a deformation of 4%.
Those mechanisms can be activated, or using direct application of heat on the shape memory alloy piece up to reaching its transformation temperature, or applying a direct electric current through this piece in order to heat it up to reaching their transformation temperature. They are based on the rupture of the fastener or retainer, being very compact. But they have the inconvenience that they can originate particles of the fractured piece that could endanger the operation of delicate equipment. Additionally, they should be substituted completely for a new use, making impossible their direct verification before operation.
Those mechanisms, based on the geometry change of the element maintaining the joint as to release the fastener or the retainer, are voluminous and heavy, having lack of compactness. They are usually used in very specialised applications that usually are not extendable to other uses.
All them highly reduce the high accelerations at high frequencies that characterises to pyrotechnic devices as they do not need any explosion, and release activation is not so sudden but more progressive than in the case of pyrotechnic devices. Nevertheless, they already have the above described inconveniences.
The objective of the present invention is to solve the problems of pyrotechnic devices and the inconveniences of the existing non-explosive actuators, by means of an actuator that fulfils the following characteristics.
a) Non pyrotechnic device (no explosion).
b) High load carrying capability, with easy preloading operation.
c) Non causing high acceleration during operation (progressive release).
d) Conceptually simple and easy operational handling to provide high reliability.
e) Reusable without any consumable.
f) Easy rearming without disassemble the device from its location, without requiring electrical disconnection.
g) Redundant activation system based in redundant electrical circuit.
h) Compatible with mechanism manual activation without requiring any configuration change.
i) Non safety critical.
j) Using non life limited components.
k) Cost effective.
In order to give solution to those problems, it was identified that a device provided with a shape memory alloy element as a trigger that liberates a mechanism able to produce the necessary geometric changes as to allow the release of a fastener or a retainer can be adequate.
Then the primary actuation can be performed by heating a shape memory alloy piece capable of recovering its preliminary shape once the temperature returns to a value below its transformation temperature. One of the most common form in the market for shape memory alloys supplying is the wire. Applying an electrical current between both ends of a shape memory alloy wire, when it reaches its transformation temperature, provides a contraction due to the change of its internal structure (from martensitic at ambient temperatures to austenitic at temperatures higher than its transformation temperature). That contraction causes a relative displacement of 4% of its length in its free end. This effect shows a very good repeatability in a high number of operation cycles, therefore it can be used as a trigger to activate the blocking system that maintains the preload of the hold down.
A very simple and efficient way to provide high load carrying capability for hardware hold down is the one formed by a ring around a retainer. Such is the case of a bolt-nut system. For that device a segmented nut (at least in three pieces) was selected in order to liberate the bolt when those segments moves radially away from the bolt central axis, releasing the joint of the equipment to be deployed or ejected. Those nut segments should move radially away a distance bigger than the bolt thread depth. Nevertheless, the number, the configuration and the geometry of segments can be changed if the fastener is substituted by a retainer, without changing the concept of the device.
In order to get that the trigger activates the radial displacement of the segments, it is necessary a mechanism that maintains the segments in position to support the preload generated by the bolt, and is capable of being released to allow the radial movement of the segments to liberate the joint.
The mechanism of the invention is based on the diameter difference that exists in a helical torsion spring between its loaded configuration and its free configuration, to provide the opening of a series of segments that compose a ring around a retainer. That is achieved by means of a helical torsion spring of an appropriate geometry as to provide a diameter variation bigger than twice the depth of the retainer groove (or twice the depth of the bolt thread), when rotating an end of the spring the adequate angle. The change from the spring loaded configuration having smaller internal diameter to the spring free configuration having bigger internal diameter, is achieved by letting one end of the helical torsion spring get free. Such segmented ring or nut allows, when moving radially away, the release of the holding part of a retainer or the threaded part of a fastener, which, up to that moment, held the equipment.
When using a retainer instead of a bolt, the geometry of the segments and their total number is directly related with the retainer geometry. In that case the helical torsion spring should provide an internal diameter change big enough as to allow the segments to move radially away, freeing the retainer to get out of its location.
The segments rest on slopes radially disposed in a piece which is rigidly joined to the device housing in order to make the segments separation easy once the helical torsion spring is released. At the other end of the segments, a piece having the possibility of free rotation, maintains the segments in position in order to allow the bolt to be threaded easily.
Also, among those slopes and being part of the same piece, a set of keys are disposed in order to avoid the rotation of the segments when the preloading is provided.
One of the helical torsion spring ends remains fixed while the other one is connected with a disk with capacity to rotate. Rotating the disk manually or using a source of external torque we can provide the necessary load to the spring and in this way reduce its diameter until hugging the segments. Once in this position, the disk has to be maintained blocked.
The trigger system that liberates the disk, and then the helical torsion spring, is based on a crown with possibility of rotation. Between disk and crown there is a cage, joined to the housing, with one or more slots to accommodate one or more balls or rollers (for clarity only rollers are mentioned from now on). Both, crown and disk are provided with axial grooves in a number identical to the number of rollers of the cage. In the crown those grooves are disposed in the internal cylindrical surface and in the disk in its external cylindrical surface. As disk and crown are susceptible to be rotated, both sets of grooves can be faced in order to allow the rollers to be located partially inside the crown or partially inside the disk.
When the device is in armed configuration, the spring is preloaded, the bolt is retained, the roller or rollers are inside the disk grooves blocking it, and the crown grooves are not faced with the cage slots preventing the rollers from moving out of the disk grooves to the crown grooves. As soon as the crown rotates up to having its grooves to the rollers, those rollers leave the disk due to the effect of the load of the helical torsion spring, and they moves to the crown grooves, allowing the disk turn and the unloading of the helical torsion spring. This allows the separation of the segments of the ring (or nut), releasing the retainer (or the bolt).
Therefore, a small displacement that faces the crown groove to the disk groove will allow the activation of the release mechanism.
There are several ways to perform the turn of the crown: manually, with an electromagnet, with an electric motor, etc.
Nevertheless, in order to get a compact design, according to one of the preferred arrangements, the rotation of the crown is performed by the action of a shape memory alloy wire, which, when provided with enough electric current to heat it until the transformation temperature, contracts due to the change of its internal structure. In this way, the primary actuator (shape memory alloy wire) transforms electric power in heat, and this in mechanical energy providing the necessary crown rotation to activate the roller trigger.
This mechanical system discharges the high bolt preload using a small actuation force. Additionally, the preload of the bolt is discharged progressively since the unloading of the helical torsion spring is not instantaneous but rather progressive turn by turn.
The use of a shape memory alloy wire allows the reuse of the mechanism, without requiring the substitution of any component, once the device is rearmed and the hold down attachment bolt preloaded. The rearming of the mechanism in ambient conditions can be carried out in seconds after having activated it.
The duration of the release can be adjusted to the necessities of each particular application, varying the wire heating speed of the shape memory alloy piece, being able to perform the actuation in few milliseconds. This is achieved varying the intensity of the electric current that goes through that component.
The rearming can be carried out in different ways. One of them is providing access to disk and crown from outside the housing through devoted slots.
Their function is to provide the possibility of facing, manually from outside, the crown groove and the disk groove with the cage slot to displace the roller and allow the disk rotation to provide, also manually from outside, the preload to the helical torsion spring. As soon as the helical torsion spring is loaded and the disk groove is faced to the crown groove, the crown has capacity to return automatically back by the action of a set of compression springs provided for that purpose. Those compression springs displace the crown groove to one side of the cage slot, blocking the disk, and leaving the apparatus ready for a new activation.
The rearming can be also performed providing lateral access to the crown, and providing access to the disk from the bolt position when it is not jet located in position.
Electrical redundancy can be provided to the primary activation electric circuit disposing two independent shape memory alloy wires to act on the same crown. In that way the reliability of the device is substantially increased as if one electrical circuit fails, there is another one available.